On retirement, right to join top club (4)
I believe the answer is:
iron
'club' is the definition.
(type of golf club)
'on retirement right to join top' is the wordplay.
'on retirement' is a reversal indicator (retire can mean to go back).
'right' becomes 'r' (abbreviation).
'to join' is an insertion indicator (inserted letters join the word).
'top' becomes 'noi' (number one).
'r' put into 'noi' is 'nori'.
'nori' reversed gives 'IRON'.
(Other definitions for iron that I've seen before include "stern" , "Smooth - driver?" , "Smoother - metal" , "medicinal preparation for the anaemic?" , "Decrease" .)